The present invention relates generally to ultrasound imaging systems, and particularly to a receiving unit of the imaging system to receive reflections of the ultrasound waves from internal portions of a body having different acoustic properties. The invention is more concerned with the focusing of the transducer array to a summing point by compensating for the diffferent times of arrival of the reflections of the ultrasound waves at the transducers.
In an ultrasound imaging system, an array of piezoelectric transducers is placed in contact with the human body and electrically energized by short pulses of ultrasonic frequency for transmitting acoustic waves into the body. The relative times of the transmission of ultrasound pulses are so selected that acoustic waves for each pulse are formed into a beam which extends in any desired direction and successively varied so that the beam is deflected to scan a sector. As the acoustic waves propagate through the body tissue, a portion of the acoustic energy is reflected as they encounter tissue of a different acoustic characteristic. Because the distances between any point of reflection and the various receiving transducers are usually different, the reflections arrive at the transducers at different times. It is therefore necessary to introduce compensating delays between each transducer and a summing point where the corresponding electrical signals derived from the transducers are combined, so that the total time between reflection of an acoustic wave from the point of reflection and the arrival of the corresponding electrical signals at the summing point is the same regardless of which transducer is involved.
U.S. Pat. No. 4,116,229 granted to R. D. Pering discloses the use of a master delay line which is tapped so that a signal injected into each tap experiences a different delay and a plurality of incremental delay elements having small amount of variable delays. Each incremental variable delay is connected between one or more of the transducers and a selected tap on the master delay line. The taps on the master delay line are connected to the transducers through the small variable incremental delays via a switching network so that the array is focused along a desired scan angle or direction and the small variable delays are changed during a scanning of that direction so as to shift the focus of the array from minimum to maximum range. The delays provided between successive taps on the master delay line are of fixed value and constitute the minimum delay corresponding to the minimum focus range. However, because of the fixed amount of master delay line, difficulty is encountered in precisely determining the amount of delay between any two successive transducers. In addition, since the incremental delay line must be provided for each transducer, the total amount of delay that is required will still be substantial when a great number of receiving transducers are employed.